1. Field of the Invention
The present invention relates to power supplies and more particularly to circuitry adapted for use in conventional power supplies of the pulse width modulated (p.w.m.) type to provide for frequency synchronization, insensitivity to temporary disturbances in the load, improved drive current response and improved high voltage shutdown response.
2. Description of the Prior Art
Many types of power supplies now make use of the well-known pulse width modulation (p.w.m.) technique to provide a signal to drive one or more power switching devices at a relatively high frequency. The p.w.m. technique makes use of a sawtooth waveform of predetermined frequency usually in the order of 20 kilohertz for purposes of generating the drive signal to the power switching devices. While the p.w.m. circuitry may be embodied by using any one of the circuit configurations well known in the art, it has become increasingly common to use devices such as integrated circuit chips to embody the p.w.m. circuitry. These chips ordinarily include an internal clock which provides the sawtooth waveform.
The supply which uses the p.w.m. technique is ordinarily connected to a load to provide power thereto. The load may, for example, be a computer controlled device which also has its own internal clock. The computer clock has a frequency which is typically in the order of several megahertz. Quite often the computer clock may be divided down in order to provide a variety of clock frequencies to the circuits contained therein. Ordinarily the internal clock of the p.w.m chip is not at a frequency which is exactly the same as the frequency of the nearest whole number submultiple of the computer clock. This whole number submultiple will be referred to hereinafter as the master clock. Having different frequencies for the master clock and the p.w.m. internal clock gives rise to signal components having frequencies which represent the sum and difference of those frequencies. These components then appear as noise within the computer and may interfere with the operation thereof. It is, therefore, desirable that the clock frequency of the p.w.m. chip be synchronized with the frequency of the master clock of the load and thereby minimize the occurrence of any such noise.
Power supplies may be used to provide d-c power to those types of loads wherein it is desirable that any temporary disturbances in the load not interfere with the steady state operation of the converter. The load which may, for example, be a computer controlled telephone switching system may contain dynamic components such as memory modules. The providing of power to these modules when the memory is to be accessed and its removal therefrom when accessing has been completed may cause a square wave of current to appear in the d-c current provided by the supply to the load. This a-c disturbance component, therefore, appears in the load. While not of long duration, it may, as described below, cause the supplies to sense a current overload condition and change from the normal and desirable, as described hereinafter, voltage regulation mode of operation to a current regulation mode of operation.
Typically the supply includes an output filter which may be embodied as the combination of an inductor and a capacitor. The current flowing in the output inductor attempts to follow those a-c variations in the load current. The inductor is not, however, capable of exactly following those portions of the a-c disturbance current. Where the disturbance current is a square wave, the current in the inductor lags behind the disturbance current for that part of the square wave where the disturbance current is rising. The inductor attempts to compensate for this lag and, therefore, overshoots (spikes) appear in the inductor current. As the output inductor is coupled to the power switching device(s) used in the supply, these spikes also appear in the switch current. The spikes may, therefore, cause the current in the power switch to exceed a predetermined upper limit. The type of coupling between the output inductor and the power switch of the supply depends on the circuit architecture used to embody the supply and may be either direct or through a transformer.
Ordinarily the supply functions in a voltage regulated mode of operation. In this mode, the supply provides an essentially constant and regulated voltage for all conditions of load up to some predetermined overload value of current in the power switch. When the current in the power switch of the supply exceeds the respective overload value, the supply switches to a current regulated mode of operation wherein the supply decreases its output voltage to provide a regulated output current. It is desirable that the associated supply not change its mode of operation in response to temporary a-c disturbances or variations in the load current as once the supply has switched to the current limited mode, it tends for reasons of stability to remain in that mode for some period of time.
A power supply may receive its input voltage from the unregulated output of another supply. The input voltage to the supply is, therefore, unregulated and may vary over a wide range. As the input voltage may vary over a wide range, it is desirable that the drive current generated to the power switch of the supply be kept constant and, therefore, independent of these variations. By keeping the drive current constant for all values of input voltage, the need to dissipate in the form of heat the higher drive losses which occur when the input voltage is at or near the high end of its range is avoided. This reduced dissipation at the high end of the input voltage range is particularly beneficial where an integrated circuit chip is used to embody the p.w.m. circuits of the supply. As is well known in the art, chips are sensitive to high heat as they are generally poor dissipators of such heat. In addition, it is also desirable that the drive current for the power switch start and stop its flow quickly in response to the p.w.m. signal.
In many applications it is desirable that power systems of the switching type include circuits which turn the corresponding supply off if the output voltage therefrom exceeds a predetermined level. Such high voltage shutdown circuits may also include a fuse rated at some predetermined amperage. In those circuits when a voltage about the predetermined level is detected, a short circuit occurs which allows more then rated current to flow through the fuse thereby causing it to open circuit.
The output voltage of the supply may rise above the over voltage level for any one of a number of different reasons. For example, energy stored in the load may, when released, cause the voltage to rise above the predetermined level. In this case, it is not desirable to open circuit the fuse as the supply itself is not the cause of the overvoltage. It is, however, desirable to turn off the supply until the output voltage falls below the overvoltage level. On the other hand, the output voltage may rise above the predetermined level because one of the circuits in the supply has failed. Ordinarily, such a failure would arise as a result of the power switch becoming short circuited. In this case, it is not possible to turn off the supply and wait for the output voltage to fall below the predetermined level. Therefore, in this case, it is desirable that the fuse be open circuited.
In those power systems of the type which include at least a first supply and a second supply, it may be desirable to turn off or shut down only the supply whose output voltage has risen above the predetermined level rather than the entire converter. For example, in such a power system the first and second supplies may be providing power either to separate loads or to parts of the same load which is of the type wherein it is possible to turn off one of the voltages without affecting the entire operation of the load. Therefore, in such a power system if an overvoltage condition should occur at the output of the second supply, it is desirable to turn off that supply but not the entire converter.